MHC class I genes are affected by a variety of exogenous stimuli which can either increase or decrease levels of expression. Although agents such as TNF and interferon are well known modulators of class I genes, many other factors also alter expression. We have observed that the thyroid stimulating hormone (TSH) specifically reduces transcription of endogenous class I genes in cultured thyrocytes. Thyrocytes normally express MHC class I, as does a rat thyrocyte cell line, FRTL-5. TSH treatment of FRTL- 5 cells decreases transcription of both TSH receptor and class I genes. This down-regulation is cAMP mediated and TSH receptor dependent. The TSH responsive element has been located within 68 bp upstream of the class I promoter, to a region which contains only canonical promoter elements. Analysis of cell extracts from normal and TSH-treated thyrocytes reveals TSH-mediated differences in the factors binding to a DNA sequence 3' to the TATAA box. Other agents are capable of modulating class I expression. Among them, insulin, hydrocortisone, and serum act as a negative regulators of class I, whereas thyroid hormone is a positive regulator. Their sites of action are distinct from those of TSH. The DNA element responsive to serum maps to a constitutive negative regulatory element, RE-lO5. Analysis of RE-lO5 does not reveal a recognizable serum response element (SRE). However, an AP1-like binding site occurs within this region.